Plastic overmolded packages with mechanically decoupled lid attach attachment

ABSTRACT

The specification describes a lidded MCM IC plastic overmolded package with chimney-type heat sink. The lid is mechanically decoupled from the chimneys by a compliant conductive polymer plug.

RELATED APPLICATION

This application is related to application Ser. No. (Crispell et al.Case 7-1-59) filed of even date herewith.

FIELD OF THE INVENTION

This invention relates to plastic overmolded packages for integratedcircuit (IC) and related devices, and more specifically to plasticovermolded packages requiring aggressive thermal management.

BACKGROUND OF THE INVENTION

A widely used form of packaging for electronic devices such as ICdevices is a plastic housing. Typically, IC chips are bonded to asubstrate and a polymer is molded over the assembly to overmold thedevice. It is common for two or more IC chips to be assembled in asingle overmolded package. Multiple chip packages are referred to asmulti-chip-modules (MCMs).

As chip size decreases in state of the art IC technology, the problem ofoverheating in IC packages becomes more severe. It is further aggravatedbecause polymers used for overmolding are poor thermal conductors. Thuswhile the plastic effectively encapsulates the devices, it traps theheat generated by the devices as well. In packages in which the IC chipis connected to the electrical terminations of the package with wirebonds, the thickness of the encapsulant must be sufficient toaccommodate the height of the wire bonds. This results in a thick“cover” of plastic over the device. Since the thermal resistance of anygiven material decreases with increasing thickness, increased thicknessfurther retards heat dissipation, all else being constant.

A wide variety of heat sink expedients have been proposed and used toaddress thermal management issues. Among these, and tailored for thetypes of packages with wire bonded IC chips, is the use of a conductive“chimney” that attaches to the top of an IC chip and becomes imbedded inthe plastic overmold. The conductive chimney conducts heat away from theIC chip, through the thickness of the plastic overmold but through thechimney itself and not the plastic overmolded material to the top of thepackage. In some package designs, the top of the chimney is affixed to alid. The lid may be made of metal, which effectively spreads the heatand conducts the heat to the external environment. In conventionaldesigns, the chimney is attached to the lid using a thermal interfacematerial (TIM). While any heat conductive material may be used for thechimney structure, silicon is preferred because of its thermo-mechanicalcompatibility with the silicon chips, low cost, availability,compatibility wih existing IC assembly equipment, and good thermalconductivity.

Device failures have been identified in these package designs.Improvement in the package design is needed to overcome these failures.

BRIEF STATEMENT OF THE INVENTION

We have studied the failure modes of IC devices with chimney-type heatsinks, and have identified in detail the causes and effects of thefailures. The two most common failure modes in these packages areresultant from the breakdown in the mechanical integrity of the chimneystack: i) the loss of attachment of the lid to the silicon chimney viathe breakdown of the TIM/lid or TIM/chimney interface; and ii) the lossof attachment of the silicon chimney to the IC device via the breakdownof the chimney-IC adhesive/chimney or chimney-IC adhesive/IC deviceinterface. When this attachment fails, the thermal conductive path fromthe IC chip to the external ambient is compromised. Among the reasonsfor detachment, a main cause is thermo-mechanical stress. Whenthermo-mechanical stresses become excessive, the lid detaches from thechimney or the chimney detaches from the IC device. We have developed aneffective approach to reducing the adverse effects of thermo-mechanicalstresses, and improving the thermo-mechanical stability of these ICpackages. Important to the improved package designs is the recognitionthat the lid should be at least partially decoupled mechanically fromthe chimney while maintaining intimate thermal coupling. This iscounter-intuitive to the tendency to approach the problem by making thebond between the chimney and the lid more mechanically robust, andconsequently more rigid. The improvements basically rely on eliminatingrobust adhesive bonding between the chimney stack and the lid andsubstituting a pliant conductive polymer cushion that allows relativemovement between the chimney stack and the lid.

BRIEF DESCRIPTION OF THE DRAWING

The invention may be better understood when considered in conjunctionwith the drawing in which:

FIGS. 1-4 are schematic views of a typical step sequence for fabricatingan overmolded IC device package with a chimney-type heat sink;

FIGS. 5 and 6 are plan views of an MCM package of four IC chips and fourchimneys that are used in this description to illustrate applicants'recognition of the failure mode in these devices;

FIGS. 7 and 8 are side views of an MCM package showing detachment modeswherein the lid separates from the chimneys, or causes failure in thebond between the chimneys and the IC chips;

FIGS. 9-12 illustrate steps, according to one embodiment, for assemblingan overmolded package with mechanical decoupling between the chimneysand the lid; and

FIG. 13 shows an alternative method for attaching the lid to thepackage.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an IC chip package comprising IC chip 14 bonded tosubstrate 11 with die attach material 16. The substrate may be anysuitable substrate material, but is typically a printed circuit board(PCB). Wire bond pads 12 and 13 are formed on the substrate in knownfashion. Referring to FIG. 2, wire bonds 21 and 22 are shownelectrically connecting the bonded IC chip 14 to the PCB. A variety ofIC chips generate significant heat during operation, and require specialtypes of heat sinking to avoid overheating and failure. For example,microprocessors are typically large IC chips fabricated with state ofthe art design rules and have very dense device packing. They posesevere thermal management issues, and consequently are usually providedwith special heat sink arrangements. One of those is shown in FIG. 3with the heat sink in the form of silicon chimney 32. In this type ofpackage, the IC chip is usually mounted on the PCB, and wire bonded forelectrical interconnection. The wire bonds are attached to edge arraysof bond pads (not shown, for clarity) on the IC chip. This leaves aspace in the center of the chip where the silicon chimney is mounted.The silicon chimney may be attached to the IC chip using a suitableattachment material 33. Attachment materials include but are not limitedto, adhesives, such as epoxies or other adhesive polymer materials orsolders. It is preferred that the adhesive material be a thermallyconductive adhesive. Many standard and commercially availableelectrically conductive adhesives are also effective conductors of heat.

The example shown is a die bonded and wire bonded device. Other forms ofIC devices, for example, flip-chip IC devices, may be usedalternatively. The IC chips are typically encapsulated, but couldcomprise bare die. Reference to IC chip is meant to include either form.In the wire bonded example shown, the height of the silicon chimney issufficient to accommodate the height of the wire bonds. The chimneyheight may be taller, or, in the case of devices without wire bonds,shorter. Silicon chimneys are usually designed for wire bonded IC chippackages.

With reference to FIG. 4, the assembly is then encapsulated in a polymerovermold 43. This protects the IC chip, the printed circuits on thesurface of the PCB, and the wire bonds. A Thermal Interface Material(TIM) 45 is then applied to the overmold, and lid 41 attached tocomplete the device. The TIM serves both as a conductive medium, toconduct heat from silicon chimney 32 to lid 41, as well as an adhesivefor retaining the lid in place. A suitable TIM for this application isAblebond 2000T®, available from Ablestick Corp. The lid 41 serves as aheat spreader and conducts heat both laterally to cooler portion of thelid located away from the silicon chimney, as well as conduction andradiation to the ambient or other system designed heat dissipationstuctures. The lid is composed of thermally conductive material, such ascopper. Typical thickness range for the lid is 0.1 mm to 1.0 mm.

FIG. 5 is a schematic representation in plan view of an MCM 51 with fourIC devices and four silicon chimneys, 53, 54, 55, 55 arranged as shownat four corners of a square. This figure is but one example of a varietyof MCM device configurations and arrangements, having fewer or moredevices and chimneys.

Chimneys 53 and 54 are spaced, center-to-center, at nominal distancea-b. When the MCM module is operated, and thermally cycled underdifferent operating conditions, for example on and off, the distance a-bwill change due to expansion/contraction of the various elements in theIC package. When a lid such as 61 is attached to the top of thechimneys, as shown in FIG. 6, the tops of the chimneys are coupled toboth the lid and the substrate, such that differential stresses due toany movement that changes distance a-b is experienced by the chimneystack and chimney/lid interface. For example, if lid 61 is copper, amaterial commonly used for lids in packages of this kind, and thepackage subjected to significant temperature changes, the copper lidwill undergo expansion/contraction dictated by the thermal coefficientof expansion, T_(c), of copper. The distance a′-b′ is determined by thatproperty, a property typically different from those that determinedistance a-b. Thus the mismatch between a-b and a′-b′ can, depending onthe thermo-mechnical properties of the materials used in the packageconstruction, cause significant shear and bending stresses in thepackage. These tend to impact the interface between the chimney and thelid and the chimney and the IC device. In severe cases this will causethe lid to detach from the package, or the chimney to detach from theIC.

Detachment often occurs between the TIM and the lid. The TIM adhereswell to the silicon chimney, but less firmly to the lid. FIG. 7 showsspace 75 developing between the lid 71 and the TIM 74.

Strains produced by differential out of plane strains, and bendingmoments, lead to either or both lid failure and chimney to IC devicefailure. These are illustrated in FIG. 8: In lid failure, the siliconchimney 84 on the left side of the figure is raised with respect to thesilicon chimney 85 on the right side of the figure. This disparity maybe the result of differences in the expansion of the chimneys, or inother elements of the package. The out of plane strains may besufficient to cause the lid 81 to completely or partially detach fromencapsulant 83 (the TIM is omitted in this figure, for clarity). The outof plane strains like that illustrated in FIG. 8 may also cause abending moment on the silicon chimney. Intuitively it can be appreciatedthat as the lid lifts on the left side in the figure (where it may bedetached from the silicon chimney 84), it tilts. This imposes a bendingmoment on chimney 85, and may cause failure of the attachment at 33,i.e. the attachment between the chimney and the IC chip.

An approach to a more robust connection between the chimney heat sinksand the lid would appear to be to increase the integrity of the adhesivebond between these elements. However, we have found that a moreeffective approach is just the opposite. The rigid attachment betweenthese elements is found to be at least partly responsible for theproblem of lid failure. Accordingly, new package structures have beendesigned wherein the lid and the chimney stack are mechanicallydecoupled. The interface between these elements, normally filled with astrong bonding adhesive, is filled with a soft conductive polymer. Thisinterface filler is sufficiently compliant and spongy, that strainscaused by the factors described above are easily absorbed and nottranslated to the lid.

One embodiment of the invention is illustrated by FIGS. 9-12. Thesefigures show a sequence of steps for implementing this embodiment. FIG.9 shows the step of molding the overmold 93. The form for this moldingoperation 95 is provided with protrusions 94 a. These protrusions arealigned with the tops of the chimneys 32, and after the overmold polymeris applied, and the mold is released, cups 94 are created in the top ofthe overmold 93. The cups align with the chimney tops as shown. Theovermold material may be chosen from a wide variety of polymers.Suitable materials are available from Cookson Electronics, Alpharetta,Ga. Typically these are thermosetting resins, and are relatively rigidafter curing. Cured epoxy materials typically have glass transitiontemperatures, T_(g), above 100° C.

Referring to FIG. 10, the cups are filled with mechanical buffers, whichpreferably are plugs of soft conductive polymer 101. The plugs 101mechanically decouple the chimneys 32 from the lid that is appliedlater. The conductive polymer may also be chosen from a wide variety ofmaterials. A suitable choice is a silicone gel, for example, Gelease™,available from Lord Corporation, Cary, N.C. This material has a T_(g) of−121° C., and a thermal conductivity of 2.3 watts per meter Kelvin(W/mK), which is about 10 times the thermal conductivity of normalplastics (0.2 W/mK). For the purpose of this invention it is recommendedthat the thermally conductive polymer have a thermal conductivitygreater than 0.7 W/mK, and have an elastic modulus and glass transitiontemperature lower than that of the overmold compound.

After application of the conductive plugs 101, the top surface of theassembly may be coated with a TIM. The TIM may be selected from avariety of materials, such as epoxy adhesives, for attaching the lid tothe package top. A suitable TIM for this application is Ablebond™ 2000T,available from Ablestik Co. This material has a room temperature modulusof 1700 MPa, and a thermal conductivity of 2.7 W/mK. FIG. 11 shows alayer of TIM 111, blanket applied to the surface of the overmold 93. TheTIM layer 111 may be applied in the form of a paste, gel, or film. A lid121 is shown applied to the TIM layer in FIG. 12. The TIM, with the lidin place, may be cured at the same time as the overmold is cured, orafter. The lid is typically in place before the TIM is cured so thatadhesion develops between the TIM and the lid during curing. Theseoperations are known in the art.

The sequence of steps shown in FIGS. 10-13 is recommended from thestandpoint of simplicity, and represent a minimum departure from thestandard method of attaching lids to IC packages having a chimney stack.However, using the conductive plugs creates an acceptable thermal pathbetween the chimneys and the lid of the package. Accordingly, it is notnecessary to use the conventional TIM. The lid may be attached by othermeans. For example, the lid may be attached using the methods describedand claimed in application Ser. No. ______ (Crispell 7-1-59).

In another embodiment of the invention, the lid is attached using anadhesive polymer, such as an epoxy adhesive, that is selectively appliedto the overmold. This option is illustrated in FIG. 13. Adhesivematerial 131 is selectively applied to the top of the overmold inregions spaced from the conductive plugs 101. The adhesive material maybe applied as a liquid, gel, film, or other suitable application. Thisadhesive material may be thermally conductive or non-conductive.However, since the adhesive in these regions does not perform anessential thermal conducting function, it is preferably a non-conductingadhesive epoxy. As indicated above normal polymers have thermalconductivity values of approximately 0.2 W/mK. Accordingly, a polymerhaving a thermal conductivity below about 0.3 W/mK would be considered anon-conductive polymer. Moreover, since the adhesive polymer in thisexample does not cover the chimneys it is not necessary for it toconform to the mechanical properties recommended earlier for theconductive polymer plugs.

In the embodiments just described at least two different polymermaterials are used in attaching the lid to the chimney stacks. One is anadhesive polymer, used to physically attach the lid to the overmold. Theother is the conductive polymer forming the plugs between the chimneysand the lid. By using two materials, the mechanical properties of thematerials can be tailored to the function required. The adhesive polymermaterial that bonds the lid to the package can be chosen for adhesiveeffectiveness, without regard to mechanical or even thermal properties.The other polymer material, the polymer that mechanically decouples thechimneys from the lid, can be optimized for mechanical and thermalproperties.

As mentioned above, the invention is applicable primarily to MCMpackages, which is intended to mean that each package contains N ICdevices, where N is at least two, with each IC device being providedwith a heat sink.

Various additional modifications of this invention will occur to thoseskilled in the art. All deviations from the specific teachings of thisspecification that basically rely on the principles and theirequivalents through which the art has been advanced are properlyconsidered within the scope of the invention as described and claimed.

1. An overmolded MCM IC package comprising: a. a substrate, b. aplurality of N semiconductor IC devices attached to the substrate, whereN is at least 2, c. a plurality of N heat sinks with a heat sinkattached to each IC device, the heat sinks having a top and a bottom,with the bottom attached to an IC device, d. a polymer overmoldencapsulating the N semiconductor IC devices and the N heat sinks, theovermold forming a top surface with the tops of the heat sinks exposed,e. a plurality of N plugs of a first polymer material selectivelyapplied to the exposed tops of the heat sinks, f. a lid attached to theovermold by a second polymer material.
 2. The package of claim 1 whereinthe second polymer material is an adhesive thermal interface material(TIM) applied over both the overmold and the plugs.
 3. The package ofclaim 1 additionally including an adhesive polymer applied selectivelyto the overmold leaving the plugs uncovered by adhesive polymer.
 4. Thepackage of claim 1 wherein the IC devices are electrically connected tothe substrate with wire bonds.
 5. The package of claim 1 wherein theheat sink is silicon.
 6. The package of claim 5 wherein the lid iscopper.
 7. The package of claim 1 wherein N is at least
 4. 8. Thepackage of claim 1 wherein the first polymer material has a Tg lowerthan the Tg of the second polymer material.
 9. The package of claim 8wherein the first polymer material has an elastic modulus lower than theelastic modulus of the second polymer material.
 10. The package of claim1 wherein the first polymer material has a conductivity greater than 0.7W/mK.
 11. The package of claim 10 wherein the second polymer materialhas a conductivity less than 0.3 W/mK.
 12. Method for the manufacture ofan overmolded MCM IC package comprising: a. attaching N semiconductor ICdevices to a substrate, where N is at least 2, b. attaching N heat sinksto the IC devices, with each IC device provided with a heat sink, theheat sinks having a bottom attached to an IC device, and a top, c.molding an overmold encapsulating the IC devices and the heat sinks,leaving the tops of the heat sinks exposed, e. applying a plurality of Nplugs of a first polymer material selectively to the exposed tops of theheat sinks, f. applying a second polymer material to the surface of thovermold, and g. applying a lid to the second polymer material.
 13. Themethod of claim 12 wherein first and second polymer materials areapplied, the lid is applied, and then the first and second materials arecured in the same curing step.
 14. The method of claim 12 wherein the ICdevices are electrically connected to the substrate with wire bonds. 15.The method of claim 12 wherein the second polymer material is appliedover the overmold and over the first polymer material.
 16. The method ofclaim 12 wherein the first polymer material has a Tg lower than the Tgof the second polymer material.
 17. The method of claim 16 wherein thefirst polymer material has an elastic modulus lower than the elasticmodulus of the second polymer material.
 18. The method of claim 12wherein the first polymer material has a conductivity greater than 0.7W/mK.
 19. The method of claim 18 wherein the second polymer material hasa conductivity less than 0.3 W/mK.
 20. The method of claim 12 whereinthe first conductive polymer is not an adhesive polymer.